1. Field of the Invention
The present invention relates to a satellite communications system and method capable of efficiently using a frequency band of satellite communications.
2. Description of the Related Art
In satellite communications, a usable frequency band of a transponder is limited and a transponder user is incurred with a charge proportional to the frequency bandwidth. An efficient use of a frequency band is therefore necessary. To this end, it has been proposed to use the same frequency band by a plurality of signals.
If different satellites are used, two signals of the same frequency band can be divided by utilizing the directivity of an antenna at each earth station. Even if the same satellite is used, two signals of orthogonally polarized vertical and horizontal waves can be discriminated by an orthogonal polarization band splitter.
However, two signals of the same polarization from the same satellite cannot be separated, posing a problem that both the signals cannot be received.
As above, in a conventional satellite communications system, the same frequency band cannot be shared for the transmission of two signals of the same polarization via the same satellite.
An example of the prior art satellite communications system is illustrated in FIG. 12.
This example shows bidirectional communications between two stations A and B (earth stations). Two signals are transmitted, one signal S.sub.A (A.fwdarw.B) being transmitted from station A to station B and the other signal S.sub.B (B.fwdarw.A) being transmitted from station B to station A. If the bands of two signals are superposed, both the signals cannot be received. Frequency division multiple access (FDMA) and time division multiple access (TDMA) have been used as a means for avoiding signal superposition.
In FDMA as shown in FIG. 12 at (1), two stations A and B transmit signals S.sub.A and S.sub.B at different frequency bands in order to avoid signal superposition and allow bidirectional communications. It is therefore necessary to occupy the bandwidth (BW(A.fwdarw.B)+(BW(B.fwdarw.A) which is a sum of the bandwidth BW(A.fwdarw.B) required for signal transmission from station A to station B and the bandwidth BW(B.fwdarw.A) required for signal transmission from station B to station A.
In TDMA as shown in FIG. 12 at (2), two stations A and B transmit signals S.sub.A and S.sub.B in a burst form at predetermined timings in such a synchronous manner that two signals are not superposed in the time domain at the transponder of a satellite. The transmission is controlled so that while the signal from station A is reaching the satellite, the signal from station B is not reaching the satellite, and vice versa. Since each station transmits a signal in a batch manner in a limited time, the necessary bandwidth increases. It is therefore necessary to occupy the bandwidth BW(A.fwdarw.B)+BW(B.fwdarw.A) which is a sum of the bandwidth BW(A.fwdarw.B) required for transmission of the signal S.sub.A from station A to station B and the bandwidth BW(B.fwdarw.A) required for transmission of the signal S.sub.B, from station B to station A. Complicated synchronizing equipments are also required.
Code division multiple access (CDMA) is also known. In CDMA, a signal to be transmitted is dispersed by using a high speed code and it is not desirable from the viewpoint of efficient frequency usage.
Conventional technology called interference compensation is known which is used as a means for correctly dividing separating a reception signal (desired wave) from a reception obstacle signal (interference signal) superposed upon the reception signal.
This technology is adopted to eliminate a reception failure in satellite communications to be caused by superposed waves in a satellite system and a terrestrial system. This technology basically relies upon an auxiliary antenna which has such a directivity that only waves of the terrestrial system can receive. Specifically, if waves (interference waves) of the terrestrial system superpose upon reception waves (desired wave) of the satellite system because of the side lobe characteristics of a satellite wave reception antenna (main antenna), another auxiliary antenna receiving only interference waves of the terrestrial system is used for cancelling the interference wave components contained in the reception waves of the main antenna.
A conventional earth station in satellite communications is not provided with a function of cancelling a local transmission signal (satellite return signal) contained in its reception signal. Therefore, a plurality of signals superposed one upon another cannot be used on the same frequency band.
Consider now that the same frequency band is made usable (band sharing) in bidirectional communications between stations A and B and the necessary bandwidth is halved at a maximum. In this case, as illustrated in FIG. 1 showing the concepts of the invention, a local transmission signal (satellite return signal) of stations A and B (earth stations) becomes an interference signal hindering normal reception, as different from conventional technologies. This interference signal comes from the same satellite as the desired wave so that it is not possible to receive only the interference signal by utilizing the antenna directivity. In conventional satellite communications, each station has no function of removing such an interference signal.